1. Field of the Invention
The present invention is in the field of CMOS operational amplifiers, and, more particularly, is directed to an output stage for such an amplifier.
2. Description of the Related Art
The gain of push-pull output stages is generally dependent both on the signal frequency and, more or less, on the signal level. In the extreme case, at low signal levels, particularly in class-B operation, so-called crossover distortion results. The signal-level-dependent gain may adversely affect the processing of audio, measurement or other signals if low distortion is of particular importance. Through such changes in the gain of the output stage, the effective degree of negative feedback around operational amplifiers, for example, becomes dependent on the signal level, so that exact linearity throughout the drive range is not ensured.
From "IEEE Journal of Solid-State Circuits," Vol. SC-22, No. 6, December 1987, pages 1082-1089, especially page 1085 and FIGS. 6a and 6b, it is known to combine the square-law characteristics of a complementary field-effect-transistor pair such that the gain of the output stage becomes independent of the signal level. Use is made of the square-law current-voltage characteristics of an n-channel transistor and a p-channel transistor: EQU I.sub.DS =.beta.(U.sub.GS -U.sub.T).sup.2,
where I.sub.DS is the drain-source current, U.sub.GS is the gate-source voltage, U.sub.T is the constant transistor threshold, and .beta. is the transistor transconductance parameter. The complementary transistor pair is driven at the respective gate terminals by a common drive potential, with the source terminals of the two transistors connected to a first fixed potential and a second fixed potential, respectively. These fixed potentials are chosen so that throughout the drive range, neither of the two transistors leaves the respective square-law current-voltage characteristic, i.e., so that both transistors remain in their respective saturation regions. Therefore, this is also referred to as class-AB push-pull operation By passing the two output currents of the complementary transistor pair through a current mirror to a high-impedance current-difference stage, a differential current is generated in the latter which has the desired signal-level-independent proportional characteristic. By means of a load resistor, this differential current can be converted to a voltage signal, e.g., in an operational amplifier.
By the application of a common drive potential to the complementary transistor pair, the drain current of one of the transistors is reduced and that of the other transistor is increased The following simplified representation illustrates the strict proportionality of the differential current I.sub.D as a function of the respective gate-source voltage U.sub.GS : EQU I.sub.D =.beta.(U.sub.GS -U.sub.T).sup.2 -.beta.(-U.sub.GS -U.sub.T).sup.2 EQU I.sub.D =.beta.(U.sub.gs.sup.2 -2U.sub.GS U.sub.T +U.sub.T.sup.2 -I.sub.GS.sup.2 -2U.sub.GS U.sub.T -U.sub.t.sup.2) EQU I.sub.D =-4.beta.U.sub.GS U.sub.T.
The drive potential is generated in an input stage which is designed in the above-cited article as a complementary cascode stage using differential technology.
The prior art circuit arrangement suffers from the drawback that the internally generated fixed potential is only stabilized via a combination of complementary diodes and a source follower, so that the impedance of the stabilizing circuit is not sufficiently low.